1. Field of the Invention
This invention relates to systems and methods for generating ozone. More particularly, the present invention pertains to tubular ozone generators used in said systems and what is conventionally the cathode or centrally located electrode thereof. Ozone generated by the teachings of the present invention has utility in many potential fields, including the purification of materials, such as water and air.
2. Background Art
Under normal conditions oxygen forms molecules having two atoms. In some circumstances, however, oxygen will form molecules that include three atoms. This form of the element is called ozone. Ozone may be produced by subjecting ordinary oxygen to ultraviolet radiation or to forms of electrical discharges, such as coronas and sparks.
The disinfecting action of ozone is well known and has been used since at least the early 1900's to destroy bacteria and certain viruses in drinking water and in air. Ozone also readily oxidized many compounds which give contaminated water its bad taste or contaminated air its bad odor. The action of ozone in purification is instantaneous, and in its reaction with contaminants, virtually no permanent residual material is formed.
The ozone molecule is unstable and tends to revert spontaneously to the two-molecule form of oxygen. This is particularly true at high temperatures. It is not, therefore, possible to store ozone in the same manner as normal oxygen or in the same manner as other materials, such as chlorine or bromine, that are used to purify air or water. Instead, when it is desired to use ozone in such purification processes, the ozone must be generated at the site where it is to be used. Then the ozone is injected immediately into the fluid being purified.
Major installations that generate ozone electrically conventionally do so by subjecting a feed gas, such as air or an oxygen rich gas, to electrical corona discharges. Using high voltages in the range of about 4,500 volts to about 20,000 volts, currents are caused to flow through such a feed gas between two electrodes. The varieties of ozone generators employing this mechanism are numerous. They include the Otto plate, the Lowther plate, and a tubular type of configuration. The Otto plate is a relatively old technology that has been in use for many years and is slowly being replaced with more efficient and reliable devices. The Lowther plate, on the other hand, is so new as to yet gain widespread acceptance. The tubular type configuration is the most common.
In a tubular ozone generator, an elongated central electrode is operated as a cathode by being subjected to a high voltage relative to an encircling outer cylindrical electrode which is grounded to function as an anode. In the elongated and usually annular reaction space that results between the central cathode and the cylindrical anode, forms of electrical phenomena are thus induced which tend with varying degrees of effectiveness to generate ozone from the feed gas in that reaction space. It has become common to enclose the central cathode terminal in a sealed dielectric envelope, usually made of glass.
Electrical phenomena related to the high voltage applied to the cathode terminal are then contained in the dielectric envelope and do not interact with feed gas to produce ozone. Instead, other corresponding electrical discharges are induced outside of the dielectric envelope by the activity therein. It is there, in an annular reaction space located between the exterior of the dielectric envelope and the interior of the cylindrical anode, that electrical phenomena generate ozone.
In the application of the technology to water purification, devices are designed to generate and inject between 2 and 6 milligrams of ozone per liter of water, with the average level of injection being about 2.4 milligrams per liter. For water treatment plants processing large volumes of water per day, the ozone produced can have daily totals ranging from 500 kilograms (about 1100 pounds) to 4500 kilograms (about 10,000 pounds). Typically, 9 to 12 kilowatt-hours of electric power are required for each pound of ozone generated. Thus, the electrical energy requirements of large water treatment plants utilizing ozone for the purification element can range from 15,000 kilowatt-hours to over a 130,000 kilowatt-hours per day on a continuing basis.
The tendency of ozone to decompose into conventional two-atom molecules of oxygen is accentuated when ozone occurs in high concentration. Additionally, in large volumes of gas containing even relatively unconcentrated volumes of ozone, the ozone molecules tend to seek each other out and thereby decompose into the normal form of oxygen. Accordingly, individual basic ozone generator units are constructed on a small scale, so that the generation product can be immediately and directly injected into the fluid to be purified. A typical basic ozone generator unit is designed to produce ozone in quantities on the order of merely grams per hour.
In large plants, such as those discussed above, a vast number of such small, individual basic units are required to cooperate together to meet the total ozone demand of the plant. Several basic generator units may be grouped together to form large generators, but those large generators are in fact an aggregation of numerous basic units operating in parallel. Not only are energy demands high under such circumstances, but with a large number of basic generating units operating, reliability or the lack thereof is a serious concern, as substantial down time and high maintenance costs can result where each individual piece of equipment experiences even occasional break downs.
In these known devices there is a tendency to combine the outputs of each of the basic generator units before using the combined output to ozonate the water. Surprisingly, combining output streams of ozone-enriched gas in this manner actually reduces the ozone concentration of the combined stream. This is a troublesome source of inefficiency.
Problems apparent in known forms of tubular ozone generators relate primarily to unreliability resulting from the malfunction of the components of the tubular generator and to inefficiency, reflected as low levels of ozone production relative to the amount of electrical energy consumed.
A number of operational phenomena combine to create these problems. For example, in prior devices, the cathode terminal contained in the dielectric envelope generally takes the form of an elongated rod that extended substantially the full length of the envelope. The buildup of heat in the device, and in the cathode terminal itself, results in metal evaporating from the cathode terminal, a process called sputtering. This causes contamination in the interior of the dielectric envelope. In addition, it results in the development of peaks and valleys on the surface of the cathode terminal. When subjected to a high electrical energies, as required for operation of the ozone generator, such peaks and valleys on the surface of the cathode terminal tended to concentrate electric charge, leading to abrupt discharges within the dielectric envelope. Such discharges have the capacity to damage or puncture the dielectric envelope.
This causes shorting to occur through the puncture directly between the cathode terminal and the anode. Such shorts have the capacity to damage the electrical supply equipment, and in particular the transformer, utilized with the ozone generator. Accordingly, known ozone generators must employ costly metal fuses for interrupting the power supply to the cathode in the case of any puncture of the dielectric envelope.
To an extent, the problems associated with cathode sputtering are lessened when an inert gas at a low pressure is introduced into the dielectric envelope. The ability of the inert gas to circulate in combination with its low mass facilitates the dissipation of heat from the dielectric envelope. In each cycle of the alternating operation of the electrical power source for the generator the inert gas has an opportunity to cool and draw heat out of the cathode rod. Furthermore, the presence of the inert gas serves to reduce evaporation and tends to disburse uniformly throughout the dielectric envelope the electrons emitted from the cathode. This expands the cathode to the walls of the envelope, so that peaks and valleys in the inner cathode itself have less consequence.
Nevertheless, even with the introduction of an inert gas into the envelope, failure problems of unacceptable frequency and severity persist. The mass of such cathode terminals taking the form of elongated rods is substantial enough that a significant amount of the inert gas is absorbed by the cathode terminal, disabling optimum functioning. The large cathode terminal resists cooling in each single cycle of the electrical power of the system, and heat naturally builds up. Pitting due to sputtering continues, causing electron concentrations, sparks, and equipment damage. Even if sputtering is reduced, the mass of the cathode terminal continues to be a major source of impurities in the atmosphere within the dielectric envelope. Also large cathode terminals absorb that atmosphere.
To counteract the presence of contaminations in the dielectric envelope, a getter material is included in the interior of the dielectric envelope for absorbing such impurities. Getter material is expensive to obtain and to install. In the case of large cathode terminals, the amount of impurities introduced into the atmosphere in the dielectric envelope can be so substantial as to overwhelm and thus eventually render ineffective any getter included therein.
An additional phenomena in known devices which contributes both to frequent component breakdown and to inefficient ozone production is the generation of nitric acid in the reaction space between the exterior of the dielectric envelope and the cylindrical anode. Nitric acid forms in the reaction area in the presence of newly-generated ozone when that ozone in the presence of humidity attacks the nitrogen in the feed gas. Thus, if feed gas to the ozone generator has an elevated dew point, the electrical phenomena in the reaction space will create droplets of nitric acid in the feed gas. As nitric acid is a conductor, such suspended droplets have a tendency to line up along magnetic lines of force and draw major electrical discharges along the path of increased conductivity that results. Such large electrical discharges tend to localize and concentrate the flow of electrons in the reaction space. Interior to the dielectric envelope this can cause corresponding electrical spikes that attack the integrity of the dielectric tube. Further, nitric acid droplets which reach the sides of the reaction space condense against the walls thereof, chemically corroding those surfaces.
Ultimately, however, the production of nitric acid in any quantity whatsoever represents an inefficient use of energy in the ozone generator. Electrical power consumed in generating the ozone that then produces the nitric acid is energy wasted in relation to desired ozone output.